This invention relates to a method for the recovery of lithium from solutions and, more particularly, to a method for the recovery of lithium from lithium-containing brines and solutions using electrodialysis.
In the recovery of lithium from ores, ore may be baked with sulfuric acid, the product leached with water, resulting lithium sulfate solution treated with lime and soda ash to remove calcium and magnesium, and lithium precipitated as carbonate. Other ore-treating methods include the so-called alkaline methods and ion-exchange methods which yield solutions of lithium as hydroxide, chloride or sulfate. These methods also include the removal of calcium and magnesium by treatment with lime and soda ash.
In the recovery of lithium from natural, predominantly chloride, brines, which vary widely in composition, an economical recovery depends not only on the lithium content but also on the concentrations of interfering ions, especially calcium and magnesium. Magnesium is particularly troublesome because its chemical behaviour in solution is very similar to that of lithium. If the magnesium content is low, removal by precipitation with lime is feasible. Evaporation and treatment with lime and soda ash, is followed by precipitation of lithium carbonate. In the case of a high magnesium content, removal with lime is not feasible and various ion exchange and liquid-liquid extraction methods have been proposed. Thus, it is obvious that, although conventional processing of ores and brines makes it possible to eliminate major portions of interfering ions, the separation of lithium from magnesium remains a serious problem.
Lithium brines have also been subjected to electrolysis or to membrane electrolysis, but usually only after the calcium and magnesium contents have been reduced to relatively low values. Therefore, electrolysis and membrane electrolysis of lithium salt solutions, usually with the object of producing a lithium compound, not only require the additional step of removing calcium and magnesium, but have the additional disadvantage of the evolution of copious amounts of gases such as hydrogen and chlorine.
It is suggested that the use of electrodialysis alone or in combination with cation exchange may overcome these difficulties to some extent and can accomplish a separation of lithium from multivalent cations such as iron, aluminum, calcium and magnesium. More specifically, in U.S. Pat. No. 3,063,924, it is stated that the removal of univalent ions and multivalent anions from aqueous solutions is easily accomplished, but the presence of multivalent cations such as calcium and magnesium causes difficulties due to the formation of deposits on membranes. Hence, multivalent cations are first removed by means of a cation exchanger whereupon calcium and magnesium deposit after which the liquid is passed through an electrodialyzing apparatus to remove a portion of at least one monovalent ion and to form a concentrated salt solution. This method still requires the prior removal of calcium and magnesium in a separate operation. G. E. Kaplan et al have reported (Chemical Abstracts, volume 60, 6507a) that good separations of lithium ions from multivalent ions, such as ferric, aluminum, magnesium and calcium ions, can be obtained at high pH in a three-compartment electrodialysis cell using unipolar and bipolar ion-exchange membranes, a nickel anode and a lead-antimony alloy cathode. The presented data show that only relatively dilute solutions have been used. It is stated that at high pH hydroxides of the multivalent cations precipitate, and that at low pH the selectivity toward these cations is considerably lowered.